The manufacture of Micro-Electro-Mechanical Systems (MEMS) such as micro-gyroscopes, micro-accelerometers, RF micro-resonators, micro-mirrors, micro-motors, micro-actuators and other such micro-devices integrating very sensitive moving mechanical parts causes a very serious challenge for a number of reasons including the facts that:                a. These very sensitive mechanical parts are typically made of silicon (polysilicon or silicon-germanium);        b. The sacrificial material underlying these mechanical parts to be released is typically silicon oxide;        c. The etch stop layer underlying this silicon oxide sacrificial layer is typically silicon nitride or silicon (polysilicon or silicon-germanium);        d. The mechanical release of the mechanical parts requires the removal of the sacrificial material in liquid or vapor HF-based chemistries;        e. A liquid HF-based solution does not allow one to perform a release in presence of a CMOS compatible Al metallization scheme;        f. A vapor HF-based approach on the other hand leads to undesirable post-release residues that are detrimental to the functionality of the MEMS devices as they block or limit the movements of the released parts.        
A method for removing those post vapor HF residues is therefore required in order to allow MEMS released structures to properly move without any potential blocking of the mechanisms. The selected method must not lead to any stiction that could obviously also be detrimental to the functionality of the devices.
A number of prior art solutions exist for removing such residues.
Liquid HF-based Chemistries
Liquid buffered HF (BHF) chemistries and non-buffered HF solutions have been used to mechanically release the sacrificial oxides underlying the silicon-based (polysilicon-based or silicon-germanium-based) structures. The problem with liquid non-buffered-HF solutions is that they quickly attack the Al-based metallization schemes which are CMOS compatible. It has been reported by Witvrouw et al that the etch rate of Al in non-buffered HF chemistries is of 800±600 nm/min. In addition, Ti displays an etch rate of 1200±600 nm/min while a TiN film demonstrates an etch rate of 0.4±0.2 nm/min. Although a TiN film is CMOS compatible, this film is known to have an intrinsic resistivity (˜50-100 μΩ-cm) much higher than the one of Al-based materials (˜2.7 μΩ-cm). TiN therefore may not be suitable for MEMS applications needing a low interconnection resistance.
On the other hand, Witvrouw et al reported slower etch rates for Al, Ti and TiN in buffered HF chemistries. The etch rates demonstrated were of 0.4±0.1 nm/min, 60±30 nm/min and 0.06±0.05 nm/minn for Al, Ti and TiN, respectively. The problem with BHF solutions is that improperly rinsed BHF released wafers will result in an undesirable precipitation of solid ammonium fluorosilicate, (NH4)2SiF6 crystals under the released mechanical parts. This clearly undesirable effect related to the use of BHF solutions is to be prevented in the manufacturing of MEMS devices.
Advanced MEMS devices need to integrate digital and/or analog CMOS control electronics and/or high voltage CMOS drivers capable of performing actuation functions. Since this CMOS electronics needs to be exposed to the release chemicals, the liquid HF-based solutions should not chemically attack the metal-based interconnection of the CMOS systems. Based on the CMOS compatibility requirements, gold (Au), although resistant to liquid HF-based solutions according to Willams et al., could not be considered as it is not CMOS compatible.
Even though a liquid HF-based solution leads to severe issues in presence of metals for CMOS control/actuation electronics, one could circumvent this problem by performing a release prior to the metallization step. This however also involves many potential issues such as:                a. potential metal residues in-between released structures;        b. Sputtering of metal is impossible due to item a above;        c. Al or other metal evaporation means a lift-off approach in conjunction with the metal evaporation techniques. Therefore, the resist strip involves liquid exposure during the remaining steps which would leave the devices prone to stiction; and        d. Low yield and cross-contamination due to potential breakage of devices along the fabrication processes that could contaminate other products.        
Such techniques are discussed in A. Witvrouw, B. Du Bois, P. De Moor, A. Verbist, C. Van Hoof, H. Bender and K. Baert, A comparison between wet HF etching and vapor HF etching for sacrificial oxide removal, Proc. SPIE Micromachining and Microfabrication Process Technology VI; September. 2000, Vol. 4174, 2000, pp. 130-141. K. R. Williams, K. Gupta and M. Wasilik, Etch rates for micromachining processing—Part II, Journal of Microelectromechanical Systems, Vol. 12, No. 6, December. 2003, 761-778.
Vapor HF-based Chemistries
Witvrouw et al. showed that Al-based metallization schemes exhibit a low etch rate of 0.03 nm/min in Vapor HF. The etch rates of Ti and TiN are of 0.19±0.02 nm/min and 0.06±0.02 nm/min, respectively. Although the above etch rates are low, one needs to remember that MEMS devices need, in some cases, to have structures released over hundreds of microns. Such excessive etch requirements therefore require excessive etch times therefore leading to an attack of the metal schemes to some extent. On the other hand, it has been demonstrated that the anhydrous HF gas-phase etching conditions used by Ouellet et al. prevent any attack of the metal structures. Even though performing a Vapor HF release in the conditions used by Ouellet et al. prevents the etch of the metal structures, it still potentially leads to the formation of undesirable fluorine-based by-products that are detrimental to the functionality of the devices as they can potentially block or limit the movements of the MEMS-released structures.
Metal-based Residues
It has been noticed by Tong et al. for example in the case of post via etch clean that AlF3 would be formed by the reaction between the gas-phase HF and the Al2O3 which is the native oxide that forms on Al upon exposure to air. The formation of AlF3 would then be formed according to the following equation:Al2O3 (s)+6 HF (g)→2 AlF3 (s)+3 H2O (g)
Although the invention from Ouellet et al described in US patent application No. 2005/0142685 addresses the removal of the toxic ammonium fluorosilicate crystals which can be formed in the presence or absence of metallization schemes on the wafer during the Vapor HF release of the MEMS mechanical structures, it does not cover the contaminants formed due to the presence of metal.
Many others have conducted studies or were granted patents related to Vapor HF processes performed in presence of metal. However, none of them (beside Tong et al.) covered the formation of residues due to the exposure of metallized devices to Vapor HF. The studies and inventions done by other individuals or groups are presented below.
Kim et al.'s invention (US patent application No. US 2004/0018683 A1) is related to the use of Vapor HF performed in presence of metal for storage (memory) devices. However, there is no mention about the potential formation of residues due to such an exposure to a Vapor HF process. In addition, Onishi's invention (U.S. Pat. No. 5,200,361) is related to the use of Vapor HF in order to clean the AlF3 and Al2O3 residues formed during the SF6 plasma etch of vias. However, it is most likely that in that case the AlF3 residues are swept away (since AlF3 is not soluble in acids) during the Vapor HF process. Tsutsui et al. (U.S. Pat. No. 5,922,623) experimented Vapor HF in presence of WSi and AlGaAs without noticing any abnormal issue. Finally, Scheiter et al. (U.S. Pat. No. 5,662,772) also used Vapor HF in presence of Al or other metal but no Post Vapor HF residue formation is discussed.
Beside that, Bergman's inventions (US patent applications No. US 2004/0069320 A1, US 2001/0029965 A1, US 2001/0027799 A1, US 2001/0017143 A1 and patents No. U.S. Pat. No. 6,830,628 B2, U.S. Pat. No. 6,240,933 B1, U.S. Pat. No. 6,240,933 B1, U.S. Pat. No. 6,843,857 B2) all focus on cleaning of Si substrates in order to remove the metallic contaminations. However, there is no discussion related to the formation of residues as such residue formation is most likely minimal. A similar application was discussed by Ma et al. but the formation of residues due to the presence of metal during the vapor phase HF etch is not covered in this publication.
As for Tong et al., they did report the formation of AlF3 from Al2O3 in presence of Vapor HF. However, they perceived AlF3 as an advantage for via resistance purposes. There was therefore no discussion related to the removal of such a by-product. For MEMS applications on the other hand, the removal of this Al fluoride compound is critical for the functionality of the devices.
It is worth mentioning that Chhabra et al were granted a patent (U.S. Pat. No. 5,089,084) in 1992 for the invention of a system in order to perform an in-situ Vapor HF/DI water cascade rinse process. Such a system was intended for Integrated Circuits as opposed to MEMS devices. Even though this invention could allow one to remove the inorganic residues formed during the Vapor HF process in presence of Al (or any other metal), this approach presents many drawbacks for MEMS applications, namely:                a. As this method is intended for CMOS processes, the wafers are not appropriately dried which would in turn lead to released structures of advanced MEMS devices displaying severe stiction problems;        b. Although the “agitation” found in a cascade-type of rinse could allow one to clear the inorganic residues formed during the Vapor HF process, this approach may not be suitable for fragile advanced MEMS devices as it could be prone to breakage due to the harsh mechanical action during a cascade rinse;        c. Such an apparatus proposed by Chhabra et al. does not exhibit any flexibility in terms of water temperature nor the possibility to use a gentle “molecular” excitation of the DI water.        
The above techniques are discussed in the following references: CRC Handbook of Chemistry and Physics, Edited by R. C. Weast, CRC Press, Inc., Boca Raton, Fla. 33431, 1979-1980 Edition; N. Chhabra and L. Gibbons, Hydrofluoric acid etcher and cascade rinser, U.S. Pat. No. 5,089,084, Feb. 18, 1992; S. Onishi, Process for preparing a semiconductor device using hydrogen fluoride and nitrogen to remove deposits, U.S. Pat. No. 5,200,361, Apr. 6, 1993; Y. Ma, M. L. Green, L. C. Feldman, J. Sapjeta, K. J. Hanson and T. W. Weidman, Vapor phase SiO2 etching and metallic contamination removal, J. Vac. Sci. Technol. B, Vol. 13, No. 4, July/August 1995, 1460-1465; J. K. Tong, J. S. Martin, T. C. Rogers and D. J. Syverson, Removal of polymeric/silicate residues and reduction of contact resistance for inter-metal via holes by vapor phase HF cleaning, Proceedings of the Fourth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, Editors R. E. Novak and J. Ruzyllo, 1996, 235-242; T. Scheiter, U. Naeher and C. Hierold, Method for the selective removal of silicon dioxide, U.S. Pat. No. 5,662,772, Sep. 2, 1997; H. Tsutsui, T. Matsumura, H. Oikawa, M. Yokoi, J. Nakamura, H. Sato and J. Mizoe, Hydrogen fluoride vapor phase selective etching methode for fabricating semiconductor devices, U.S. Pat. No. 5,922,623, Jul. 13, 1999; A. Witvrouw, B. Du Bois, P. De Moor, A. Verbist, C. Van Hoof, H. Bender and K. Baert, A comparison between wet HF etching and vapor HF etching for sacrificial oxide removal, Proc. SPIE Micromachining and Microfabrication Process Technology VI; September 2000, Vol. 4174, 2000, pp. 130-141; E. J. Bergman, Methods for cleaning semiconductor surfaces, U.S. Pat. No. 6,240,933 B1, Jun. 5, 2001; E. J. Bergman, Methods for cleaning semiconductor surfaces, US patent application No. US 2001/0017143 A1, Aug. 30, 2001; E. J. Bergman, Methods for cleaning semiconductor surfaces, US patent application No. US 2001/0027799 A1, Oct. 11, 2001; E. J. Bergman, Methods for cleaning semiconductor surfaces, US patent application No. US 2001/0029965 A1, Oct. 18, 2001.; S.-Y. Kim, K.-T. Lee and Y. P. Han, Method of manufacturing storage nodes of a semiconductor memory device using a two-step etching process, US patent application No. US 2004/0018683 A1, Jan. 29, 2004; E. J. Bergman, Methods for cleaning semiconductor surfaces, U.S. patent application No. US 2004/0069320 A1, Apr. 15, 2004; E. J. Bergman, Methods for cleaning semiconductor surfaces, U.S. Pat. No. 6,830,628 B2, Dec. 14, 2004; E. J. Bergman, Methods for cleaning semiconductor surfaces, U.S. Pat. No. 6,843,857 B2, Jan. 18, 2005; and L. Ouellet, G. Migneault and J. Li, A new anhydrous HF release process for MEMS, US patent application 2005/0142685.
Non-metal Based Residues
The reaction between Silicon Nitride (Si3N4) used as an etch stop during the release process and the Vapor HF leads to the formation of ammonium fluorosilicate residues as follow:Si3N4 (s)+16 HF (g)→2(NH4)2SiF6 (s)+SiF4 (g)
This ammonium fluorosilicate residue is thermally unstable and decomposes at a temperature higher than 100° C. as follows:(NH4)2SiF6 (s)→NH4HF2 (s)+SiF4 (g)+NH3 (g)
For the (NH4)2SiF6 and the NH4HF2 compounds, those residues can be eliminated by performing and ex-situ or an in-situ annealing step or by operating the Vapor HF release at a triple point at a pressure of less than 40 Torr and a temperature above 100° C. as reported by Ouellet et al, supra. However, Ouellet et al. did not address the possibility that their invention could leave some fluorine-based residues behind.